The invention relates to acoustic echo cancellation systems, including active acoustic attenuation systems and communication systems. The invention arose during continuing development efforts relating to the subject matter of U.S. Pat. No. 5,033,082, incorporated herein by reference.
In one aspect of the invention, a fully coupled active echo cancellation matrix is provided, canceling echo due to acoustic transmission between zones, in addition to cancellation of echoes due to electrical transmission between zones as in incorporated U.S. Pat. No. 5,033,082. In the latter patent, a communication system is provided including a first acoustic zone, a second acoustic zone, a first microphone at the first zone, a first loudspeaker at the first zone, a second microphone at the second zone and having an output supplied to the first loudspeaker such that a first person at the first zone can hear the speech of a second person at the second zone as transmitted by the second microphone and the first loudspeaker, a second loudspeaker at the second zone and having an input supplied from the first microphone such that the second person at the second zone can hear the speech of the first person at the first zone as transmitted by the first microphone and the second loudspeaker, a first model cancelling the speech of the second person in the output of the first microphone otherwise present due to electrical transmission from the second microphone to the first loudspeaker and broadcast by the first loudspeaker to the first microphone, the cancellation of the speech of the second person in the output of the first microphone preventing rebroadcast thereof by the second loudspeaker, and a second model cancelling the speech of the first person in the output of the second microphone otherwise present due to electrical transmission from the first microphone to the second loudspeaker and broadcast by the second loudspeaker to the second microphone, the cancellation of the speech of the first person in the output of the second microphone preventing rebroadcast thereof by the first loudspeaker. In the present invention, there is provided a third model cancelling the speech of the first person in the output of the first microphone otherwise present due to acoustic transmission from the second loudspeaker in the second zone to the first microphone in the first zone, and a fourth model canceling the speech of the second person in the output of the second microphone otherwise due to acoustic transmission from the first loudspeaker in the first zone to the second microphone in the second zone. The present invention has desirable application in those implementations where there is acoustic coupling between the first and second zones, for example in a vehicle such as a minivan, where the first zone is the front seat and the second zone is a rear seat, and it is desired to provide an intercom communication system, and cancel echoes not only due to local acoustic transmission in a zone but also global acoustic transmission between zones, including in combination with active acoustic attenuation.
In another aspect of the invention, there is provided a switch having open and closed states, and conducting the output of a microphone therethrough in the closed state, a voice activity detector having an input from the output of the microphone at a node between the microphone and the switch, an occupant sensor sensing the presence of a person at the acoustic zone, and a logical AND function having a first input from the voice activity detector, a second input from the occupant sensor, and an output to the switch to actuate the latter between open and closed states. This feature is desirable in automotive applications when there are no additional passengers for a driver to communicate with.
In another aspect of the invention, an input to a model is supplied through a variable training signal circuit providing increasing training signal levels with increasing speech signal levels or increased interior ambient noise levels associated with higher vehicle speeds. This is desirable for on-line training noise to be imperceptible by the occupant yet have a sufficient signal to noise ratio for accurate model convergence.
In another aspect of the invention, a noise responsive high pass filter is provided between a microphone and a remote yet acoustically coupled loudspeaker, and having a filter cutoff effective at elevated noise levels and reducing bandwidth and making more gain available, to improve intelligibility of speech of a person in the zone of the microphone transmitted to the remote loudspeaker. In vehicle applications, the high pass filter is vehicle speed sensitive, such that at higher vehicle speeds and resulting higher noise levels, lower frequency speech content is blocked and higher frequency speech content is passed, the lower frequency speech content being otherwise masked at higher speeds by broadband vehicle and wind noise, so that the reduced bandwidth and the absence of the lower frequency speech content does not sacrifice the perceived quality of speech, and such that at lower vehicle speeds and resulting lower noise levels, the cutoff frequency of the filter is lowered such that lower frequency speech content is passed, in addition to higher frequency speech content, to provide enriched low frequency performance, and overcome objections to a tinny sounding system.
In another aspect, there is provided a feedback detector having an input from a microphone, and an output controlling an adjustable notch filter filtering the output of the microphone supplied to a remote yet acoustically coupled loudspeaker. This overcomes prior objections in closed loop communication systems which can become unstable whenever the total loop gain exceeds unity. Careful setting of system gain and acoustic echo cancellation may be used to ensure system stability. For various reasons, such as high gain requirements, acoustic feedback may occur, which is often at the system resonance or where the free response is relatively undamped. These resonances usually have a very high Q factor and can be represented by a narrow band in the frequency domain. Thus, the total system gain ceiling is determined by a small portion of the communication system bandwidth, in essence limiting performance across all frequencies in the band for one or more narrow regions. The present invention overcomes this objection.
In another aspect of the invention, an acoustic feedback tonal canceler is provided, removing tonal noise from the output of the microphone to prevent broadcast thereof by a remote but acoustically coupled loudspeaker.
Other features and aspects are provided, as will be understood in the following description.